Iraqi security forces advanced into militant-held Tikrit on Tuesday in an assault aimed at revitalizing flagging efforts to retake the city, officials said.

Security forces began the attack on Tuesday morning and have succeeded in retaking government facilities in the city's south.

Iraqi forces pressed a campaign Saturday to retake militant-held Tikrit, clashing with Sunni fighters nearby and pounding positions inside the city with air strikes in their biggest counter-offensive so far.

A senior officer said security forces were coordinating with Washington, which has deployed military advisers to Iraq as it battles jihadist-led militants who have overrun large parts of five provinces north and west of Baghdad.

Dozens of prisoners were on the loose on Friday after militants attacked a prison in the Iraqi city of Tikrit, leaving at least 13 policemen dead, officials said.

The violence at the prison comes after al-Qaida's Iraqi front group announced a campaign to regain territory and said it aimed to help its jailed members escape.

A bomb placed outside a mosque frequented by provincial officials in the Iraqi city of Tikrit killed 17 people and wounded 50 after the main weekly Muslim prayers on Friday, a security official said.

Two members of the provincial council and a police officer were among the wounded, the official said.